The present invention relates to an improved method for producing all-E astaxanthin dimethyldisuccinate of the formula I.

Astaxanthin itself is a sought-after dye which is used primarily as a feed additive for pigmenting farmed salmon.

A very efficient method for producing astaxanthin is the double Wittig olefination of the symmetrical C10 dialdehyde of the formula II with two equivalents of the corresponding 015 phosphonium salt of the formula III as described, inter alia, in Helv. Chim. Acta 64, 7, 2445 (1981).

This reaction can be carried out, for example in dichloromethane using methanolic sodium methoxide solution as base (cf. also WO 2007128574) or by heating the components of the formulae II and III in 1,2-epoxybutane.
For improving the storage stability of astaxanthin in fish food, various diesters of astaxanthin are proposed in WO 03/066583, wherein, inter alia, astaxanthin dimethyldisuccinate of the formula I is mentioned.
For producing the diesters of astaxanthin, according to WO 03/066583, crystalline astaxanthin is reacted in an inert solvent with a carboxylic acid chloride or a carboxylic acid anhydride in the presence of an organic base (WO 03/066583, page 7, lines 3 to 20). In the esterification of astaxanthin with a free acid, the reaction proceeds in the presence of a dehydrating reagent (WO 03/066583, page 7, lines 21 to 28). Astaxanthin dimethyldisuccinate of the formula I was obtained according to example 5 in WO 03/066583 (page 13, lines 13 to 25) in a yield of 49.9% and a purity of 79.3%. By subsequent recrystallization from methylene chloride/methanol, the diesters could be obtained in a purity of 98% (after HPLC).
A disadvantage of this procedure is that the starting material used for the esterification is crystalline astaxanthin. The production of crystalline astaxanthin as an intermediate for the esterification means considerable production expenditure for crystallization, filtration, washing, drying, packaging, storage and metering the solid in the subsequent stage. In addition, in the crystallization of astaxanthin, losses of yield occur owing to the residual solubility of the product of value in the mother liquor.